The present invention relates to an image forming apparatus for forming images on sheets of printing paper and, more particularly, an image forming apparatus capable of printing images efficiently on a large number of print sheets.
FIG. 10 is a side view of a conventional image forming apparatus that functions as a mimeograph machine. A thermally prepared original sheet is stuck around the outer circumference of a drum 30 constituting a part of a printing section. Print sheets P are fed continuously between the drum 30 and a press roller 30a, whereby the image of the original is mimeographed onto the print sheets.
The print sheets P are stacked on a sheet feed table 31. The sheets are picked up one by one, from a top sheet to bottom, by a scraper roller 32 and a pickup roller 33. The two rollers constitute a primary sheet feed mechanism 37. The picked up sheets are fed toward the drum 30.
Between the drum 30 and the pickup roller 33 is provided a timing roller 40 which, driven intermittently, constitutes part of a secondary sheet feed mechanism 38. The timing roller 40 in operation transports each print sheet P for printing by the drum 30 at an appropriate timing synchronous with the drum revolutions.
As outlined, the typical mimeograph machine comprises the primary and secondary sheet feed mechanisms 37 and 38. The two-stage sheet feed arrangement is required for the mimeograph machine to operate at a higher speed than other printers such as copiers. The primary sheet feed mechanism 37 extracts the print sheets P slowly and one by one from the sheet feed table 31; the mechanism makes sure that it does not pick up inadvertently two or more sheets at a time. The secondary sheet feed mechanism 38 feeds the print sheets P to the drum 30 rotating at a high speed, the rate of print sheet feed being synchronized with the drum revolutions. The mimeograph machine may be switched stepwise from low to high printing speeds.
A belt 46 is held taut over the drum 30, a plurality of idlers 35, and the shaft 45a of a motor 45. The motor 45 rotates the drum 30. The driving force of the motor 45 is also used to activate the timing roller 40 intermittently in an intermittent driving mechanism 50.
The intermittent driving mechanism is structured as follows: a guide roller cam 51 and a timing roll cam 52 are attached fixedly to the shaft 45a of the motor 45. One end 53a of a substantially fan-shaped gear 53 is in contact with the guide roller cam 51, and one end 54a of a timing lever 54 is in contact with the timing roll cam 52.
The fan-shaped gear 53 swings around a shaft 53c when the guide roller cam 51 is rotated. The other end 53b of the gear 53 is engaged with a guide roller 55. While in swing motion, the gear 53 has its end 53b rotate the guide roller 55 clockwise. A rotating timing roll cam 52 causes the other end 54b of the timing lever 54 to swing intermittently, thereby actuating the timing roller 40 toward the guide roller 55.
As the guide roller 55 rotates in the forward direction, the timing roller 40 is urged toward the guide roller 55. The motion feeds each print sheet P toward the drum 30 at an appropriate timing.
One disadvantage of the conventional mimeograph machine is the complexity described above of the intermittent driving mechanism 50 that drives intermittently the timing roller 40 making up a part of the secondary sheet feed mechanism 38. Another disadvantage is that the machine is subject to inherent constraint on the speed of its printing. That is, the intermittent driving mechanism 50 is structured fixedly in such a manner that one print sheet P is fed in synchronism with a single revolution of the drum 30. Thus the only way to increase the printing speed is to raise the revolutions of the drum 30, i.e., the rotating speed of the motor 45. The procedure involved has its obvious limitations.
Because one revolution of the drum 30 results in the printing of only a single print sheet P, only one original mimeograph sheet of a predetermined size is allowed to be attached over the drum 30. A thermally prepared original plate, which is diverse in size, may be substantially small compared with the mimeograph sheet but must be integrated with the latter for printing of one print sheet P alone per revolution of the drum 30. In other words, even if the size of an original plate is so small that two or more plates might be accommodated in one original mimeograph sheet, the conventional mimeograph machine does not permit that sort of use.
If, say, two plates were to be formed in one original mimeograph sheet and if two print sheets P were to be fed in synchronism with each revolution of the drum 30, a far larger number of print sheets would be printed and the printing speed would be boosted significantly. As it is, the conventional machine still uses the intermittent driving mechanism 50 with its fixed operation for feeding print sheets P; the machine is thus limited to the way it works as described above.
With the conventional mimeograph machine, print sheets P may be adjusted in vertical image forming position by moving the two idlers 35a (FIG. 10) up and down. Moving the idlers 35a in this manner adjusts the synchronism between the rotation of the drum 30 and the intermittent operation of the intermittent driving mechanism 50. This in turn adjusts the feed operation of print sheets P relative to the original mimeograph sheet so that the image forming position on the print sheet surface will be regulated vertically. Adjustment of the vertical image forming position involves the chores of mechanically relocating the idlers 35a, which makes the accurate fine-tuning of the image position all the more difficult.